Differences of bcl-2 protein expression between Merkel cells and Merkel cell carcinomas

The bcl-2 gene, originally identified in B-cell lymphomas, encodes for proteins which may assume oncogenic functions by blocking apoptosis. Bcl-2 proteins are broadly distributed among various tissues, including epithelial ones. Within the skin, bcl-2 is strongly expressed in melanocytes, but its further distribution is yet unclear. The Merkel cells, neuroendocrine-epithelial cells of the skin, are present within the epidermis and hair follicles, mostly nerve-associated, and are believed to be postmitotic and long lived. Possibly they give rise to the malignant Merkel cell carcinomas. In the present study we investigated the bcl-2 expression on the protein level by means of immunohistochemical techniques including double confocal laser scanning microscopy, as well as on the RNA level by RT-PCR techniques, in Merkel cells, Merkel cell carcinomas, and cell lines. Merkel cells were identified by double staining for cytokeratins 20 or 8/18. We demonstrate that fetal epidermal and dermal Merkel cells are immunostained for bcl-2 protein, most of them clearly weaker than melanocytes. Adult Merkel cells also express bcl-2 protein very heterogeneously, mostly weak. In contrast, Merkel cell carcinomas are usually strongly positive for bcl-2 protein with some degree of heterogeneity. This is different from malignant melanomas in which bcl-2 expression is reduced as compared to normal melanocytes. Bcl-2 gene expression was also shown for Merkel cell carcinoma cell lines on both the mRNA and the protein level. Possibly bcl-2 protein expression is downregulated during the life span of Merkel cells, arguing that they may succumb to a certain cell turnover. The comparably high bcl-2 protein level in Merkel cell carcinomas may reflect peculiar biological and clinical characteristics.